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Understanding Lumens in Lighting
Years ago, in the ‘old school’ sales days, customers were more concerned about the ‘wattage’ of lightbulbs, flashlights work lights etc. As technology has basically ‘reinvented’ the lightbulb with L.E.D.’s, (Light Emitting Diodes), Eco-Lights, (Fluorescent Bulbs), and S.M.D.’s, (Surface Mounted Devices), and with the gradual phasing out of standard incandescent bulbs, the new ‘buzz word’ in lighting is ‘Lumens’.
The lumen, (symbol: lm) is the SI, (System International), derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all electromagnetic waves emitted, independent of the eye's ability to perceive it. Lumens are related to lux in that one lux is one lumen per square meter.
The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.
The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square meter, lights up that square meter with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square meters, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.
A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution.
On September 1, 2010, European Union legislation came into force mandating that lighting equipment must be labelled primarily in terms of luminous flux (lm), instead of electric power (W). This change is a result of the EU's Eco-design Directive for Energy-using Products. For example, according to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 700–750 lm.